1. Field of the Invention
The present invention relates in general to a method of coating a metal member and in particular to a new and useful method for the codeposition of chromium and silicon for coating iron-base (Fe-base) alloys using a pack cementation process.
2. Description of the Related Art
It is common in the field of coating metals or alloys to use a pack cementation process. Generally, a pack cementation process is a modified chemical vapor deposition process which consists of heating a closed or vented pack to an elevated temperature for a specific amount of time during which a diffusional coating is produced on a metal. The closed or vented cementation pack is protected from oxidation by an inert or reducing atmosphere. The cementation pack usually consists of the metal or alloy member or substrate that is to be coated surrounded by the elements to be deposited usually in the form of a powder masteralloy, a halide activator salt and a powder filler. An inert gas, such as argon, or else hydrogen is used to surround the pack. Once the pack is heated to a sufficiently elevated temperature, the activator salt reacts with the elements of the masteralloy to form metallic halide vapors. The metallic halide vapors diffuse to the substrate or metal surface through the gas phase in the porous pack. At the substrate surface, a reaction step results in the formation of a protective coating on the metal surface; the surface reaction may be somewhat complex, involving adsorption, dissociation, and/or surface diffusion of the molecular species.
The goal of coating a workpiece, this term meaning to include either a pure metal or a metal alloy object or substrate, is to provide a corrosion-and oxidation-resistant coating which will enhance the use and service life of the workpiece in severe environments.
Currently, most commercial cementation coating processes involve the deposition of single elements such as aluminum, chromium or silicon. There are, however, some processes that are advertised that involve deposition of two elements such as chromium and aluminum, or aluminum and silicon, that involve a complex two-step procedure that consists of incorporating these elements sequentially into the metal or substrate surface. However, the resulting elemental distributions are not optimal for oxidation/corrosion resistance.
A common problem encountered in the current commercial pack cementation coating processes results when chromium is introduced by the coating process to a workpiece that contains significant carbon content. A chromium carbide layer will form and subsequently block the inward diffusion of the protective chromium constituent into the workpiece. Therefore, those metals which do contain carbon and are subjected to a coating process involving chromium will have a resulting coating that is not optimally effective against the corrosive elements in many severe environments.